1. Field
The present disclosure relates to a MIMO (Multiple-Input, Multiple-Output) communication method as communication having multiple inputs and multiple outputs, and in particular, to a so-called massive MIMO communication method using a large number of transmission antennas.
2. Description of the Related Art
Currently, the amount of wireless communication by portable terminals is explosively increasing with the spread of smart phones. In the future, it is assumed that communications among devices without human intervention are on the rise, and it is expected that the amount of traffic will increase to as much as a thousand times to ten thousand times the current amount in and after 2020. Accordingly, researches are actively carried out in new communication methods. That is to say, in addition to LTE (Long Term Evolution) that has currently come into commercial use, and LTE-A, researches are being made in methods having higher efficiency in frequency usage, and proposals are about to be made as the fifth generation methods of 3GPP.
As one of powerful techniques among them, massive MIMO (massive MIMO) is provided.
As illustrated in FIG. 8, a base station BS has a large number (Nb pieces) of transmission antennas (Ant#1 to Ant#Nb), and a communication system that performs communication with a plurality of (the number N of) terminals (UE#1 to UE#N) through channels h00 to h0(Nb−1), . . . , h(Nb−1)0 to h(Nb−1)(N−1) at the same time is constituted. The number Nb of transmission antennas of the base station BS is, for example about 100 pieces, and the number of mobile terminals is, for example about 10 units.
With such a construction, although the frequency use efficiency has been affected by influences of channel noise and fading that changes rapidly so far, it is possible to keep the frequency use efficiency stable without being practically affected by those influences. Also, the channel capacity dramatically increases compared with related-art MIMO. For this reason, massive MIMO is the most dominant technique as the next generation new technique.
In this regard, concerning SU-MIMO (Single User-MIMO) and MU-MIMO (MultiUser-MIMO) as multiple-input and multiple-output communication, a proposal of a disclosure is made in a prior application U.S. Ser. No. 14/178,898 by the present applicant. In this proposal, code multiplexing with different amplitudes is applied to an orthogonal frequency division multiplexing (OFDM: Orthogonal Frequency Division Multiplexing) method so that it makes possible to reduce N pieces of reception antennas that are necessary for related-art (N×N) MIMO to one piece. Thereby, it becomes possible to perform MIMO high-speed communication without increasing the number of antennas of a terminal.
A massive MIMO method was introduced in about 2010 by academic papers, and research results aimed for practical applications were eagerly released in the middle of 2013. The details are described, for example, in Non Patent Literature 1, 2. By this, in the case where the number of terminals is Nv, and the number of antennas of the base station BS is Nb, the channel matrix H becomes Nv×Nb. Here, at the time of transmission from the BS (downlink, DL), when multiplying the channel matrix H by its complex conjugate transpose H* (“*” represents complex conjugate transpose), the reception data of each terminal becomes H·H*· (transmission data). H·H* is a matrix called a correlation matrix G.
Here, non-diagonal elements of the correlation matrix G (that is to say, H·H*) become smaller compared with diagonal elements if the correlation between different channels is low. Using this correlation matrix G, it is possible for a terminal to receive a signal for the own terminal without performing operation, such as diagonalization, or the like as a result. Also, noise superimposed on a channel is reduced by the correlation matrix G, because auto- and cross-correlation of the noise itself is low. As a result, it becomes possible to perform reception that is less subject to the influence of SN.